FIELD OF THE INVENTION
In a variety of data transmission applications, it is necessary to control the access to a transmission channel jointly used by a plurality of data sources. This is the case for example during the concentration of the packet data (HDLC frames) of the individual ISDN subscribers of a digital telephone switching system onto PCM time slots at 64 kbit/s (Bd channels). There, the HDLC frames of a plurality of assemblies (data sources) have to be transmitted via one Bd channel (transmission channel).
In order to control access authorization, a method is used at the present time in which an OPEN FLAG is transmitted before each HDLC frame (data frame) and a CLOSE FLAG is transmitted after each HDLC frame, and in which an uninterrupted sequence of logic one values (MARK) is always transmitted as IDLE code. This prior art method has the disadvantage that the IDLE code (idle bit sequence) that is transmitted is not a sequence of FLAG signals, for example "01111110", but rather MARK, even though the IDLE code "FLAG" is prescribed in many transmission protocols. A data stream transmitted by means of the known method must therefore be post-processed in a suitable manner in order to satisfy the specifications of many transmission protocols.